There have been proposed a large number of detection systems that manipulate the orbits of secondary particles that are generated from a specimen, and discriminate the secondary particles by energy or emission cone angle (an angle defined between the optical axis of a primary electron beam and a direction of the emitted secondary particles) to detect the secondary particles.
For example, JP-A No. 2002-110079 discloses a scanning electron microscope that is capable of discriminating the secondary electrons lower in the energy and reflected electrons lower in the energy from each other, and discriminating the reflected electrons by the emission cone angle to detect the reflected electrons. The scanning electron microscope includes an ExB (orthogonal electromagnetic field generator) that deflects the electrons lower in the energy to a detection unit direction, and a first detection unit that detects the deflected electrons at an electron gun side of an objective lens. Also, at the electron gun side of the ExB, the scanning electron microscope includes a second detection unit that detects the secondary particles higher in the energy which have passed through the ExB.
The secondary electrons lower in the energy which have been emitted from the specimen are deflected to the detection unit direction by the ExB, and are detected. Also, parts of the reflected electrons higher in the energy, which are larger in the cone angle at the time of emission, are allowed to collide with the first electrode that is disposed between the objective lens and the ExB so as to be converted into signal electrons lower in the energy to detect the signal electrons as with the secondary electrons. On the other hand, parts of the higher energy reflected electrons, which are smaller in the cone angle at the time of emission, are converted into the signal electrons by the second electrode that is disposed at the electron gun side of the ExB to detect the signal electrons by the second detection unit.
The first detection unit detects both of the secondary electrons lower in the energy and the reflected electrons higher in the energy and larger in the cone angle. In this situation, a voltage is applied to the first electrode to change the polarity or intensity of the voltage. This makes it possible to change the mixture ratio of the two kinds of electrons which are detected by the first detection unit. For example, when only the secondary electrons are intended to be detected, a positive voltage is applied. As a result, the occurrence of the signal electrons is suppressed to detect only the secondary electrons. On the other hand, when only the reflected electrons are intended to be detected, a negative voltage is applied. As a result, a potential barrier is produced, and the secondary electrons are returned in the specimen direction and not detected. When an absolute value of the negative voltage is set to an appropriate value, the secondary electrons and the reflected electrons are mixed together at a constant mixture ratio so as to detect the secondary electrons.
Also, JP-A No. 2004-221089 discloses a scanning electron microscope which is capable of discriminating the secondary electrons with the low energy and the reflected electrons with the high energy from each other, and detecting the secondary electrons of high yield. The scanning electron microscope has an accelerator tube having a positive potential applied to the interior of the mirror body, and has two detection units in the interior of the accelerator tube.
The secondary particles that are generated from the specimen are pulled up to the interior of the mirror body by the accelerator tube, and directed toward the electron gun direction. In this situation, the locus is different according to the energy of the secondary particles, and the secondary electrons are focused on a location closer to a specimen table as the energy of the secondary electrons is lower.
The energy discrimination is detected as follows. The first detection unit that detects the secondary electrons lower in the energy is disposed on the optical axis where the secondary electrons are sufficiently dispersed after having being converged, to detect the second electrons. In this situation, because the reflected electrons higher in the energy pass through the center hole of the first detection unit, it is possible to detect only the secondary electrons.
The second detection unit that detects the reflected electrons higher in the energy is disposed on the optical axis where the reflected electrons are sufficiently dispersed after having been converged. An aperture is disposed on a location on which the reflected electrons are focused so as to enable the secondary electrons that have passed through the secondary electron detection unit to be blocked. The aperture travels on the optical axis, or the inner diameter of the aperture changes, thereby making it possible to optimize the yield point of the reflected electrons while the secondary electrons are appropriately blocked.
Further, JP-A No. Hei03-49142 discloses a method in which there is used an auxiliary lens for converging the secondary electrons to the electron gun side of an objective lens in order to enhance the detection efficiency of the secondary electrons in an SEM using a retarding method that enables high resolution observation at a low acceleration. In the method, only the secondary electrons that have been accelerated by the retarding voltage are converged at the electron gun side of the auxiliary lens by the auxiliary lens, and the detection unit is placed in a region where the secondary electrons are dispersed after having been converged, so as to detect only the secondary electrons. In order to reduce an influence of the auxiliary lens on the primary electron beams, the auxiliary lens is arranged at a position where the primary electron beam is converged.